Since a metal colloid shows a color peculiar to the particle size and the kind of metal, a measuring tip for optical analysis which utilizes the metal colloid as an optical filter is known (see, for example, Japanese Unexamined Patent Application, First Publication No. Hei 10-160737). The metal colloid used in this measuring tip is obtained by previously preparing a metal colloidal dispersion and mixing the colloidal dispersion with a silane coupling agent, thereby to introduce an amino group as a functional group into the surface of the colloid. However, when the metal colloid is formed and then the surface protective agent is introduced into the surface of the colloid, the protective agent may not be sufficiently introduced because of the deposit which has already been present on the metal surface of the colloid. Since the metal colloid is synthesized in an aqueous system, the surface protective agent is influenced by hydrolysis and thus stability of the colloid deteriorates. Furthermore, the amino group to be introduced into the amino group is used as a functional group for a protein or enzyme, and therefore the amino group is located outside and the siloxane bond of the protective agent is located on the surface of the colloid. Therefore, according to surface properties of colloidal particles, adhesion between the protective agent and the colloidal particles may become insufficient and thus the metal colloidal film is unstable.
In addition, as a metal colloid used as a conductive ink or a material of a conductive coat, a highly conductive aqueous metal colloidal solution containing an organic component is known (see, for example, Japanese Unexamined Patent Application, First Publication No. 2001-325831). However, this metal colloid is also produced by the aqueous reaction and is obtained by mixing with the organic component after forming a metal colloid and therefore have the same problems described above.
Also it has hitherto been known to use a metal colloid as a colorant of a coating composition or glass. For example, it is known that a metal colloid is prepared by reducing a metal compound in the presence of a high-molecular weight pigment dispersant (see, for example, Japanese Unexamined Patent Application, First Publication No. Hei 11-80647). However, this method also includes a major specific example wherein a metal colloid is formed by the aqueous reaction and therefore have the same problems described above. Furthermore, a coexisting polymer protective colloid is a pigment dispersant and is not obtained by bonding a protective agent comprising a silane coupling agent with the surface of colloidal particles.
Furthermore, there is known a method of mixing chlorauric acid with a protective polymer to form a gold colloid, the method using a protective polymer having an amino group in the end or side chain portion opposite the surface of metal particles (see, for example, Japanese Unexamined Patent Application, First Publication No. 2000-160210). This method is intended to produce a gold colloid without using a reducing agent such as sodium borohydride which is commonly used, and the reductive action of a protective polymer is utilized. However, in this case, since the protective agent is a polymer, the resulting colloid contains a lot of organic chains and is insufficient in heat resistance.
A gold ink, which has hitherto been sold and used as a gold powder, has been obtained by treating the surface of a flat shaped brass powder (copper-zinc alloy powder) with a saturated fatty acid having 16 to 22 carbon atoms and used for lithographic printing. However, a lithographic printing ink has high viscosity and is not suited for use as an ink having low viscosity which is used for gravure printing. Therefore, there is disclosed a gold powder for gold ink which is prepared by coating 100 parts by weight of a flaky brass metal powder having an average particle size of 10 μm or less with 0.1 to 2 parts by weight of a saturated fatty acid having 14 to 22 carbon atoms and 0.1 to 2 parts by weight of a fatty acid amide having 14 to 22 carbon atoms while mixing (see, for example, Japanese Unexamined Patent Application, First Publication No. 2001-19872) as measure which can attain the same specular gloss as that in case of gravure printing in the lithographic printing and also can exert the same effect as that of a smooth paper in case of using a paper having no smooth surface. In Japanese Unexamined Patent Application, First Publication No. 2001-19872, when using, as a printing ink, a gold ink obtained by mixing a gold powder having an average particle size of 10 μm or less prepared by a mechanical crushing method with a predetermined amount of the saturated fatty acid and the fatty acid amide, a metal specular glossy film is obtained.
Also, a method of preparing a metal colloid with a silica film by a heat treatment using an amino group-containing alkoxysilane is known (see, for example, Extended Abstracts of 66th Fall Meeting of the Chemical Society of Japan, pp. 322 and Proc SPIE Sol-gel Optics III, Vol. 2288, pp. 130-139).
However, the amino group-containing silane used in this method is used so as to promote the production of a colloid from chlorauric acid as a raw material, and is not used as the protective agent. According to this method, since colloidalization is conducted by a heat treatment, properties of the colloid produced vary depending on the temperature and stable permeation and absorption performances. Moreover, hydrolysis of a sol-gel solution is promoted by an acid containing in a raw material in an alkoxide, and thus lifetime of the solution tends to be shortened and furthermore the solution is unstable.